In general, a secondary battery, which can be recharged and used repeatedly, is used as an assembled battery in which a plurality of single cells is assembled depending on the load. When a single cell is overcharged during the charging of such an assembled battery, the life of the single cell could be adversely affected. Therefore, it is necessary to prevent each single cell from being overcharged during the charging process.
When a single cell is overcharged, some changes are brought about in the single cell including an increase in charge voltage over a predetermined voltage; an abnormal rise in temperature; or transformation of the surface by swelling, which results when a polar plate inside the single cell swells and accordingly pushes out on the surface. Monitoring these changes, therefore, allows the overcharging of a single cell to be detected.
There are several approaches for preventing the overcharging of a single cell. One of them is to monitor a single cell charge voltage on the side of a charging device during the charging of an assembled battery, in which when the charge voltage exceeds a predetermined voltage, the charging of the assembled battery will be terminated. In case of any failure of the charging device, however, it becomes impossible to stop charging on the basis of a single cell charge voltage.
Another approach is to detect the surface temperature of a single cell with a temperature sensor and the like, in which, when a surface temperature exceeds a predetermined temperature, the sensor will detect overcharging. Usually, however, there is a difference in temperature between the inside and the surface of a single cell, and also the surface temperature of a single cell is subject to changes of ambient temperature. Accordingly, it becomes difficult to grasp the definite temperature inside a single cell enough to shut off charging.
Another approach, as disclosed in publication of the unexamined Japanese patent application 2000-353552, is to house single cells in a container in which a space is left between the inner wall of the container and the surface of a single cell, and in which a switch is installed so as to operate by being pressed. According to this configuration, if a single cell is overcharged and then excess transformation by swelling occurs on the surface of the single cell, the switch will end up being pressed by the swollen single cell surface and this will lead to termination of charging. Thus, this approach allows the problems previously described to be prevented from occurring and makes detection of overcharge possible.
However, these types of assembled batteries usually require a ventilation space between single cells to help dissipate the heat from them. Therefore, the use of the above described configuration, where installation of the switch results in the need of a space between the inner wall of the container and each single cell, leads to an increase in size, and consequently concerns about a considerably large place for installation.
The present invention has been developed in view of such circumstances. It is an object of the invention to provide both an assembled battery device and a power supplying device using this assembled battery device so as to make the overcharging of a single cell surely detectable and avoid an increase in battery size.